(1) Field of the Invention
The present invention relates to wireless data communications systems, and more particularly to measuring the distortion in a received signal and pre-distorting the transmitted signal to compensate for the measured distortion in the received signal.
(2) Description of the Related Art
Please refer to co-pending application Ser. No. 08/387,246 filed on Feb. 13, 1995 by N. Gopalan Nair, entitled USE OF CONTROL CHANNEL INFORMATION TO ENHANCE DATA THROUGHPUT OF AN INTEGRATED CELLULAR COMMUNICATION SYSTEM, and co-pending application Ser. No. 08/387,245 filed on Feb. 13, 1995 by N. Gopalan Nair, entitled INTEGRATED CELLULAR DATA/VOICE COMMUNICATION SYSTEM WORKING UNDER ONE OPERATING SYSTEM, where both co-pending applications are assigned to the same party as the present invention.
Modems are data communications devices that provide connections for computers into the public switched telephone network. A sending modem converts digital signals generated by computers to analog signals for transmission over the telephone system's limited bandwidth analog transmission lines. A receiving modem somewhere else in the telephone network converts the received analog signals back to digital signals and supplies them to the receiving computer.
Communication between two computers by modem is preceded by a signal exchange that establishes the parameters of the communication. For example one of the parameters established is data transmission speed. A second set of parameters exchanged deals with distortion in the transmission channel. All transmitted signals are susceptible to distortion, where distortion is defined as the received signal not being an exact replica of the transmitted signal. Some distortion is introduced by the medium through which the transmitted signals pass such as signal attenuation. Some distortion is due to outside interference such as background noise, storms, emf from machinery, multi path transmission interference, etc. All of these factors result in distortions in the ideal amplitude and phase characteristics of the transmission channel. With data transmission, distortion is more of a problem than with voice since the human brain can deal with a considerable degree of distortion and missing data and still capture the message. Not so with data transmission between computers. To deal with this problem, the prior art added circuitry to the receiving side of the modem that allowed the modem to compensate for some degree of distortion in the transmission channel. The level of distortion was determined by the modems at the beginning of each call by passing signals with known wave forms back and forth and measuring the distortion. Once the distortion was determined, distortion correction could be set and communicated to each modem. However, once set, distortion correction parameters remain constant for the duration of the call. This is, of course, an adequate solution if the communication channel consists of a circuit through a land based telephone network. This is true since once a circuit is established in a land based network, it does not change appreciably over the time of a typical data transmission.
The problem of distortion correction changes however if one or both of the communication points is mobile. In this case, the distortion characteristics of the transmission channel can change--sometimes radically--in very short periods of time. The most recent mobile data communication systems have attempted to deal with this problem by adding adaptive distortion compensation to modem receivers. Adaptive distortion correction is accomplished by adding logic and circuitry to the modem. The circuitry and logic monitors the incoming signal and makes periodic adjustments to the compensation electronics.
These schemes are useful and improve data integrity over a mobile data communication network. However, there are many times when such compensation is not adequate to deal with the degree of distortion present. When this is true, either the error rate increases or the data throughput decreases.